Recycling of vat and reactive dyed textile waste to new ...

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Haslinger, Simone; Wang, Yingfeng; Rissanen, Marja; Lossa, Miriam; Tanttu, Marjaana; Ilen,Elina; Määttänen, Marjo; Harlin, Ali; Hummel, Michael; Sixta, HerbertRecycling of Vat and Reactive Dyed Textile Waste to New Colored Man-Made CelluloseFibers

Published in:Green Chemistry

DOI:10.1039/c9gc02776a

Published: 01/01/2019

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Published under the following license:CC BY

Please cite the original version:Haslinger, S., Wang, Y., Rissanen, M., Lossa, M., Tanttu, M., Ilen, E., Määttänen, M., Harlin, A., Hummel, M., &Sixta, H. (2019). Recycling of Vat and Reactive Dyed Textile Waste to New Colored Man-Made Cellulose Fibers.Green Chemistry, 21(20), 5598-5610. https://doi.org/10.1039/c9gc02776a

https://doi.org/10.1039/c9gc02776a

https://doi.org/10.1039/c9gc02776a

Green Chemistry

PAPER

Cite this: Green Chem., 2019, 21,5598

Received 6th August 2019,Accepted 18th September 2019

DOI: 10.1039/c9gc02776a

rsc.li/greenchem

Recycling of vat and reactive dyed textile waste tonew colored man-made cellulose fibers†

Simone Haslinger, a Yingfeng Wang,a Marja Rissanen,a Miriam Beatrice Lossa,b

Marjaana Tanttu,c Elina Ilen,c Marjo Määttänen,d Ali Harlin,d Michael Hummel a

and Herbert Sixta *a

The successful recycling of colored textile waste and reuse of respective dyes would represent a major

milestone of global efforts to reduce the environmental impact of the textile industry. The chemical up-

cycling of dyed pre- and postconsumer cotton waste is promoted by studying the spinability and color

fastness of seven vat and reactive dyes (i.e. Indanthren Blue BC 3%, Indanthren Red FBB coll, Indanthren

Brilliant Green FBB coll, Levafix Brilliant Red E-4BA, Levafix Blue E-GRN gran, Remazol Brilliant Blue R

spec, and Remazol Black B 133%) during dry-jet wet spinning. Apart from the fabrics dyed with Levafix

Brilliant Red E-4BA, all samples dissolved in 1,5-diazabicyclo[4.3.0]non-5-ene actetate, a superbase based

ionic liquid, and could be converted to new colored man-made cellulose fibers. It was found that there is

a clear discrepancy between the recyclability of dyed pre- and postconsumer cotton waste, resulting in

significantly higher fiber properties up to tenacities of 59.8 cN/tex and elongations of 13.1% in case of the

latter. All recycled fibers displayed a noticeable color change in the CIELab space (ΔE = 8.8–25.6)

throughout the spinning process. Despite these deviations, almost all fibers and demo fabrics produced

thereof exhibited bright colors that can be reused in textile industry. Only Remazol Black B 133% did not

sufficiently translate to the new textile product. The wash and rubbing fastness of the fabrics knitted from

the regenerated fibers was superior to the dyed waste fabrics mainly because of the homogenous distri-

bution of the dyes along the fiber cross-section.

Introduction

Fast fashion has transformed the perception of clothing froma valuable, long-lasting product into a short-term use, disposa-ble good.1 Europe only recycles 25% of its textile waste, thevast majority is incinerated or landfilled. Cotton is considereda renewable resource, but its ecological footprint is equal oreven larger than the impact of polyester.2 When cotton ishowever recycled, its carbon footprint can be cut into half asfurther cotton cultivation is minimized.3

Various strategies for the conversion of cotton waste gar-ments into new value added products have been developedover the past decades. They can mainly be divided into chemi-cal and mechanical approaches. In the latter, cotton waste isshredded, opened, and carded to reclaim single cotton fibers.4

Dependent on their final application, these can be pretreatedor bleached before they are respun into new yarns.5 However,strategies to preserve and reuse the original color are urgentlyneeded to avoid the generation of further effluents such aswastewater from dyeing.6 Textile dyeing can require up to 150liters of water per kilogram of fabric. In developing countries,where most of the production takes place and where adequateenvironmental legislation is often lacking, the wastewater isfrequently discharged unfiltered into waterways. 700 000 tonsof synthetic dyes are produced worldwide. Up to 200 000 tonsof these dyes are lost to effluents every year during dyeing andfinishing operations of textiles, due to the inefficiency of thedyeing processes.7

In the recent past, some mechanical recycling technologieshave emerged, aiming at color retention during the reuse oftextile waste. The mechanical recovery of cotton circumventsboth, cotton cultivation and dyeing, by sorting the raw-material prior to the yarn manufacturing. The waste material

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9gc02776a

aDepartment of Bioproducts and Biosystems, Aalto University, P. O. Box 16300,

Espoo, FI-00076 Aalto, Finland. E-mail: [email protected],

[email protected], [email protected], [email protected],

[email protected] of Chemistry and Biology, Hochschule Fresenius, University of Applied

Science, Limburger Str. 2, 65510 Idstein, Germany.

E-mail: [email protected] of Design, Aalto University, Otaniementie 13, 02150 Espoo, Finland.

E-mail: [email protected], [email protected] Technical Research Centre of Finland Ltd, P.O. Box 1000, Espoo, FI-02044 VTT,

Finland. E-mail: [email protected], [email protected]

5598 | Green Chem., 2019, 21, 5598–5610 This journal is © The Royal Society of Chemistry 2019

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is subjected to a process similar to ginning to clean and openthe cotton fibers.8 By blending different colors, a broad spec-trum of yarns can be obtained, which show properties similarto conventional yarns produced from virgin materials.9 Thedrawback is that the raw-material consists of mainly pre-consu-mer cotton waste, whereas only small amounts of postconsu-mer cotton can be added to the yarn production.4,9 Nowadays,75% of pre-consumer cotton is reused,3 while postconsumerwaste is more problematic to recycle due to coloring, impuri-ties and degradation induced by sun light irradiation, mechan-ical abrasion, and laundering.10,11

Therefore, chemical recycling offers more options for post-consumer waste. Recent studies have shown that cotton fromtextiles can be used to produce biogas,12 cellulose nano-particles,13 aerogels14 or microcrystalline cellulose.15 When itcomes to textile industry, a cradle-to-cradle approach is moredesirable, implying that textile waste is dissolved and re-spuninto new fibers. In 2016, we demonstrated the conversion ofwhite post-consumer cotton to Lyocell type fibers by dry-jet wetspinning using the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-ene actetate [DBNH] [OAc] as a direct solvent. The procedurefirst requires assessing the viscoelastic properties of the result-ing solution, which are entirely determined by the cottonwaste material. Dependent on how heavily the fabric was usedand washed, its properties might be suitable for dry-jet wetspinning without any further pretreatment.16 In many cases,the degree of polymerization of cellulose, however, needs to belowered by hydrolysis to a target value to fit the requirementsof the spinning process.16,17 Besides the adjustment of macro-molecular properties, the pretreatment can also involve purifi-cation steps such as bleaching and the removal of metals.18 Ableached product has so far been more preferable on a com-mercial scale due to hygiene and aesthetic reasons, although itincreases the consumption of chemicals and water, alsodemanding subsequent re-dyeing. It is evident that theseapproaches do not sufficiently comply with the principles ofgreen processing. A recycling strategy is hence only truly sus-tainable if the original color of a waste fabric can be translatedto the new product directly.

Preliminary studies have shown that it is, in principle, poss-ible to dissolve dyed textile waste and to spin new coloredfibers thereof. This can be achieved via different systems suchas the viscose process,19 the NMMO-based Lyocell process,20

alkali/urea,21 or a mixture of 1-butyl-3-methylimidazoliumacetate and DMSO.22 Similarly, [DBNH] [OAc] can be used torecycle colored textile waste.23 However, there are more than10 000 different dyes used in textile manufacturing and identi-fying the exact chemical compound(s) in textile waste is nearlyimpossible.24 Therefore, more systematic studies are needed todiscern categories of textile dyes that are suitable for chemicaltextile recycling. Respective dyes have to maintain their mole-cular integrity when exposed to the chemical environment andthermal impact of the fiber spinning process. Further, theyshould not alter the rheological properties of the spin dope,which could impair its subsequent spinability.25 Finally, thedyes have to coagulate with the cellulose to avoid their

accumulation in the coagulation bath, which would affect thesolvent recycling procedure and compromise the overalleconomy of the process. This recycling route, in which the dyeis co-dissolved with the polymer, can be compared to spindyeing, which is frequently used for synthetic polymers. Spun-dyed fibers have a considerably reduced environmental impactcompared to classical textile dyeing.26 Initial studies havereported the spin dyeing of Lyocell fibers through the additionof inorganic pigments,27 different colorants and their precur-sors28 to the spinning solution. Also vat-dyed pulp mixed withundyed pulp was found to result in a good colorfastness.29,30

Vat dyes are water insoluble in their oxidized form, but dis-solve in water once reduced. This enables them to be precipi-tated on the fiber surface by a sequence of reduction and oxi-dation reactions. Moreover, vat dyes, such as Indanthrenes, arethermally stable due the presence of anthraquinone units. Inthe 1960s, 40–50% of all cellulosics were dyed with vat dyesbecause of their excellent resistance to acid, alkali, and bleach-ing. Nowadays, vat dyeing is still used for textile products thatrequire to withstand a large number of washing cycles orindustrial laundering, including work wear, householdarticles, and outdoor equipment. With the development ofmore stable and cost-effective reactive dyes, vat dyes lost theirdominant position on the market. Today, the global share ofreactive dyes amounts 20–30%, which involves the dyeing ofmost cellulose products. Reactive dyes consist of four units,most commonly referred to as solubilizing, chromophore,bridging, and reactive groups.31 Comparable to vat-dyes, reac-tive dyes with anthraquinone chromophores are considered tobe more resistant to laundering, elevated temperatures, andbleaching than azo colorants.32,33 The reactive groups of reac-tive dyes can mainly be divided into halo heterocycles thatform a covalent bond with cellulose via nucleophilic substi-tution, and activated vinyl compounds such as vinyl sulfonesthat attach to cellulose through addition reactions.31 Finally,the reactivity of these units and the nature of the chromophoredetermine the color fastness of a garment throughout its life-cycle as well as its subsequent recyclability.

As mentioned before, no studies have yet systematicallyinvestigated the role of different dye classes in a chemicalupcycling process for cotton based textile waste, which none-theless appears essential to prevent an extensive consumptionof chemicals and water. For the first time, this study targets tocreate a better understanding of the recyclability of dyedcotton fabrics within a dry-jet wet spinning process, using asuperbase based ionic liquid such as [DBNH] [OAc] (Ioncell™),by assessing the impact of seven commercial colorants ontothe production of man-made cellulose fibers. We tested threevat dyes (Indanthren Blue BC 3%, Indanthren Red FBB coll,Indanthren Brilliant Green FBB coll) and four reactive dyes,among them two vinyl sulfone dyes (Remazol Brilliant Blue Rspec, Remazol Black B 133%), and two halo quinoxaline dyes(Levafix Brilliant Red E-4BA, Levafix Blue E-GRN gran). Pre-and post-consumer cotton waste was used as textile substrate.

The white fabrics were dyed, ground, and their intrinsicviscosities were measured. This approach was chosen because

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the exact dye formulation of waste textiles is usually unknown.In this way, it was possible to relate the process behavior andcolor stability to the particular dye during the entire spinningprocess.

The pre-consumer fabric was pretreated with electron beamirradiation, sulfuric acid and endoglucanases to adjust thedegree of polymerization. The effect of the pretreatment wasassessed based on the molar mass distributions, and com-pared to the properties of untreated dyed post-consumerwaste. Subsequently, the samples were subjected to dry-jet wetspinning and if spinable processed to knitted fabrics. Besidesthe determination of tensile properties and fiber orientation,the color change of the dyes throughout the spinning processwas monitored, together with their subsequent fastness towashing and rubbing. Ultimately, final prototype garmentswere manufactured from staple fibers spun directly from solu-tions of worn dyed textile fabrics to demonstrate that thedyeing of white textile waste corresponds to that of genuinedyed textile waste in terms of recyclability.

Materials and methodsRaw material

White pre-consumer cotton (100%) was purchased fromIisalmen Kangastukku (Finland). The product name wasPainopohja (printing base) and consisted of a plain weavestructure (148 g m−2). White postconsumer hospital bedsheets (100% cotton) were supplied by the Uusimaa HospitalLaundry (Uudenmaan Sairaalapesulo Oy, Finland). As allsamples displayed distinct viscoelastic properties, the limitingviscosity of each bed sheet was determined separately accord-ing to the SCAN-CM 15:88 standard of cellulose in cupri-ethyle-nediamine (CED) solution. Moreover, all samples were groundusing a Wiley Mill and their dry-matter content wasdetermined.

Dyes and dyeing procedures

Indanthrene Red FBB coll (C.I. Vat Red 10), IndanthrenBrilliant Green FBB coll (C.I. Vat Green 1), Levafix Brilliant RedE-4BA (C.I. Reactive Red 158), and Levafix Blue E-GRN gran(C.I. Reactive Blue XX) were kindly provided by DyStar(Germany). Indanthren Blue BC 3% (C.I. Vat Blue 6), RemazolBrilliant Blue R spec (C.I. Reactive Blue 19) and Remazol BlackB gran 133% (C.I. Reactive Black 5) were purchased from A.Wennström Oy (Finland). The chemical structures of the dyescan be found in Fig. 1.

All fabrics were dyed in 1 kg batches. During vat-dyeing,water (20 l) was heated in a kettle (50 °C) and Na2S2O4 (3 g ml−1),25 wt% sodium hydroxide (NaOH) and Na2SO4·10H2O(12 g l−1) were added. Once the respective Indanthrene dye(2% of the dry weight of the samples) had been dissolved andreduced, the cotton fabrics were soaked in the dyeing bath for45 min under constant stirring. In order to enhance the oxi-dation of the dyes, the fabrics were subsequently rinsed withwater followed by hydrogen peroxide (2 ml l−1) until no leach-

ing of color could be observed anymore. Reactive dyeing wasconducted in a washing machine (Esteri Pesukoneet Oy,Vantaa, Finland) using a 60 °C/20 l program with the followingdyeing sequence: Prewetting + gentle spin–dyeing–rinsing–boiling–rinsing–spin. As in the previous procedure, the reac-tive dyes amounted 2% of the dry mass of the cotton samples.The sample to liquor ratio was 1 : 20 (kg l−1). Na2SO4·10H2O(50 g l−1) was added to increase the affinity of the dyes tocotton, while Na2CO3 (9 g l−1) was used to provide an alkalineenvironment. After dyeing, all fabrics were air-dried.

Pretreatment of pre-consumer cotton

Waste cotton was pretreated in three different ways using acidhydrolysis (A), electron beam irradiation (e-beam), andsequences of enzymatic treatment with endoglucanases (EG)and acids (A) to reduce the degree of polymerization (DPv) toan intrinsic viscosity of 450 ± 50 ml g−1 (DPv = 1050 ± 100).

(A): Sulfuric acid (H2SO4, 95–97%, Merck or Sigma) wasdiluted to 0.27 M and heated to 82 °C. Subsequently, groundcotton was added in a sample to liquor ratio of 3 : 100 andtreated 7–9 min to yield a target viscosity of 450 ± 50 ml g−1.After the treatment, the sample was washed with cold de-ionized water to stop the reaction.

(E-beam): The electron beam pretreatment was conductedby means of a 10 MeV accelerator at Leoni Studer AG

Fig. 1 Chemical structure of the vat and reactive dyes used.

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(Switzerland). A series of different electron beam dosages(5–30 kGy) were tested to reach the required viscosity.

(EG-A, A-EG): The sequences of endoglucanase and acidtreatments were carried out by VTT (Finland). The shreddedvat-dyed cotton material was treated with an endoglucanase-rich enzyme Ecopulp R (AB Enzymes, Finland) at a consistencyof 25 wt% for 4 h at 50 °C. Prior the treatment, the cottonsample was dispersed in water to 2 wt% and subsequentlydewatered to enhance the enzymatic treatment. The enzymedosage was 10 mg protein per g of cotton pulp. After the reac-tion had been stopped (90 °C, 30 min), the fibrillated materialwas cooled and rinsed with deionized water. The acid treat-ment was carried out for 2 h at 90 °C at a consistency of10 wt% (oven dry cotton/water). The pre-heated cotton materialwas dispersed in water and 0.45 wt% (based on the oven dryweight of cotton) of H2SO4 (0.5 M, FF-Chemicals Ab) wereadded.

After the reaction had been stopped by diluting the mixturewith deionized water to 5 wt% consistency, the sample wasdewatered and washed with cold deionized water.

Molar mass distribution (MMD)

The molar mass distribution (MMD) of both dyed pre- and post-consumer samples was analyzed by gel permeation chromato-graphy applying one precolumn (PLgel Mixed-A, 7.5 × 50 mm)and four analytical columns (PLgel Mixed-a, 7.5 × 300 mm). Theset-up was equipped with a refractive index detector (ShodexRI-101). The samples were dissolved in 90 g l−1 LiCl/N,N-di-methylacetamide (DMAc) via a solvent exchange procedure(H2O/acetone/DMAc). The system was operated at a temperatureof 25 °C and a flow rate of 0.75 ml min−1 using 9 g l−1 LiCl/DMAc as an eluent. The injection volume was 100 µl. Pullulanstandards (Mw 343–708 000 Da) were used as calibration stan-dards employing a direct-standard calibration model.34

[DBNH] [OAc]

1,5-Diazabicyclo [4.3.0] non-5-ene (99%, Fluorochem, UK) wasneutralized with glacial acetic acid (100%, Merck, Germany).35

Dissolution studies

In order to analyze the solubility of the dyes in the ionicliquid, small amounts of dye were suspended in [DBNH] [OAc]and stirred under constant heating (80°, 1 h). A drop of theresulting mixture was then placed onto a glass slide andobserved under a heating microscope (80 °C, Zeiss Axio scopeA1, Zeiss AG, Germany). The same procedure was also con-ducted for the spinning dopes prepared from the dyed wastecotton fabrics to evaluate the effect of dyes on the dissolutionof the respective cotton substrate.

Dope preparation and rheology

After grinding, the dyed fabrics were dissolved in [DBNH][OAc] (80 °C, 90 min) to obtain a 13% cellulose solution usinga vertical kneader system. After dissolution, undissolved par-ticles were removed from the solution by press filtration(80 °C, 1–2 MPa, metal filter fleece, 5–6 µm absolute fineness,

Gebr. Kufferath AG, Germany). The resulting dope was thenshaped into a cylindrical mold and stored upon solidification(1–7 days, 5–8 °C). The viscoelastic properties of the spinningsolution were evaluated on an Anton Paar Physica MCR 300rheometer and on an Anton Paar Physica MCR 302 rheometer(Austria). A dynamic frequency sweep test (100–0.1 s−1) wasused to obtain the complex viscosity η* and the dynamicmoduli (G′, G″) as a function of the angular frequency ω.Assuming the validity of the Cox-Merz rule, an apparent zeroshear viscosity η0* of the spinning solutions could be calcu-lated, and the crossover point of G′ and G″ was determined toyield additional information on the cellulose substrate.36

Dry-jet wet spinning

The cellulose solution was spun into filaments on a custo-mized lab-scale piston unit (Fourne Polymertechnik,Germany). The spin dope was heated according to its rheologi-cal properties and extruded through a 200-holes spinneret(0.1 mm diameter, 0.02 mm capillary length). While passing aca. 1 cm air gap, the filaments were stretched with a draw ratioof 12 (DR = vtake-up/vextr.). After spinning, the fibers were cutinto staple fibers (4 cm), opened, washed with water (80 °C,2 h), and air dried.

Tensile properties and birefringence

The properties of all fibers spun from colored post-consumercotton waste including tenacity (cN/tex), elongation at break(%), and linear density (dtex) were determined on a Favigraphdevice (Textechno, Germany). Test parameters: 20 cN load cell,10 mm gauge length, 10 mm min−1 test speed, 100 mg preten-sion, fiber count 20. All samples were measured in conditioned(20 °C, 65% relative humidity) and wet state (10 s wetting priorto the testing). The Young’s Modulus was calculated from theinitial slope of the individual stress strain curves. The fibersproduced from pretreated, dyed pre-consumer cotton were ana-lyzed on Vibroskop and Vibrodyn 400 devices (Lenzing,Instrument, Austria). Test parameters: 20 mm gauge length,20 mm min−1 test speed, 100 mg pretension, fiber count: 10.

The total orientation of the all fibers was estimated using apolarizing light microscope (Zeiss Axio Scope, Zeiss AG,Germany). Fibers showing an average linear density were tapedto a glass slide, and the birefringence angle was measured forthe bottom, middle, and top part of the fibers. Assuming adensity of 1.5 g cm−3, the total orientation factor ft could becalculated by dividing Δn by the maximum birefringence valueof cellulose (0.062).37

Color coordinates and color difference

The optical properties of all samples before and after spinningwere assessed by CIELab coordinates using a 10 °C observerand a standard illuminant D65 (SpectroScan, GretagMacbeth,Germany). The difference in color ΔE was calculated from thefollowing equation,

ΔE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðΔL*Þ2 þ ðΔa*Þ2 þ ðΔb*Þ2

q



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where ΔE denotes the total difference between two colors inthe CIELab space, ΔL* the difference in lightness, Δa* thedifference in the red-green axis, and Δb* the difference in theyellow-blue axis.

Yarn spinning and tensile testing

The staple fibers were soaked into a spin finishing solution(50 °C, 5 min) consisting of Aflian CVS and Leomin PN(Archroma, Switzerland). The ratio of Aflian CVS and LeominPN was 4 : 1, in total amounting 0.25% of the dry mass of thefibers. The sample to liquor ratio was 1 : 20. After drying, thefibers were opened by a fiber opener (Trash analyzer281C,Mesdan, Italy). The opened fibers were carded (CardingMachine 337A, Mesdan, Italy for the fibers spun from post-consumer waste or Automatex, Italy for the fibers spun frompre-consumer cotton). The carded web was formed to a sliver,drafted (Stiro Roving Lab 3371, Mesdan, Italy) and doubledtwice, and formed into a roving. The yarn was spun with aring-spinning machine (Ring Lab 82BA, SER.MA.TES, Italy, forthe fibers spun from post-consumer waste, or Ring Lab,Mesdan, Italy, for the fibers spun from pre-consumer cotton).The yarn twist direction was Z and twist per meter 700. Bothtenacity (cN/tex) and elongation at break (%) of the spun yarns(single thread) were measured on a MTS 400 tensile tester(MTS, USA) with a 50 N load cell, a gauge length of 250 mm,and a test speed of 250 mm min−1. Before the measurement,the yarns were conditioned (20 ± 2 °C, 65 ± 2% relative humid-ity) and their average linear density (tex = g per 1000 m) wasdetermined from 20 m skeins. The plied yarn was produced bytwisting two threads together (DirectTwist®, Agteks, Turkey).The ply twist direction was S with 300 twists per meter.

Demonstration products

The yarn produced from recycled, Indanthren Blue BC 3%dyed pre-consumer cotton was employed to produce the babyjacket demonstrator. The base fabric of the baby jacket wasinterlock-knit (127 g m−2) using a single thread yarn (18 tex Z700), and the sleeve cuffs were 2 × 1 rib-knit using two com-bined threads together (18 tex Z 700 × t0). The fabrics wereknitted by a flatbed weft knitting machine (Stoll CMS ADF32W E7.3 multi gauge, Stoll, Germany). Both materials werefinished solely by steam ironing before sewing. The bindingmaterial in the edges was a commercial knitted viscose,narrow fabric band. The cotton based sewing yarns are typicalin knit garment production. The garments were sewed in aprototyping laboratory using industrial-like sewing machinery.No finishing treatments were applied after sewing.

The blue, plied yarn (221 tex Z 700 × 2 S 300) produced fromrecycled Remzol Brilliant Blue R spec dyed post-consumercotton was used for the scarf demonstrator. The knit structurewas mixed purl and lace and the same flatbed knitting machinewas employed as in the baby jacket demonstrator.

For color fastness testing, single jersey fabrics (150 g m−2)were knitted (Stoll CMS ADF 32W E7.3 multi gauge, Stoll,Germany) from plied yarns spun from pre- and post-consumercotton.

Wash fastness

The color fastness of the dyed fabrics and the fibers after spin-ning was tested according to EN ISO 105-C06 (color fastness todomestic and commercial laundering). The specimens weresewn together with a DW-multifiber adjacent fabric. If thespecimen was loose fibers, the mass of loose fiber was equal toone-half of the combined mass of the adjacent fabrics (multifi-ber and non-dyeable fabrics). The size of cotton fabric speci-men was 100 mm × 40 mm. The mass of regenerated cellulosicfabric specimens was similar to cotton fabric specimens. Thewash liquor was prepared by dissolving 4 g l−1 of detergent(AATCC 1993 Standard Reference Detergent WOB) in deionizedwater. The test method was A1S, where the temperature was40 °C and the liquor volume was 150 ml. The number of steelballs was 10 and the washing time was 30 min. The specimenswere washed in the Linitest equipment (Original Hanau,Germany). At the end, the specimens were rinsed twice for1 min in two separate 100 ml portions of water at 40 °C. Thespecimens were air-dried. The change in color of the specimenand the staining of the multifiber fabric were assessed usingthe grey scales under D65 illumination. The highest value ofthe grey scale is 5 (no color change or staining) and 1 is theworst (cf. Fig. S1 and S2 in the ESI†).

Rubbing test

The color fastness to rubbing of the dyed fabrics and theregenerated cellulosic knitted fabrics was tested according tothe EN ISO 105-X12:2016 standard. The specimens wererubbed 20 times (10 times to and 10 times fro) with a dryrubbing cloth (desized, bleached cotton cloth) and with a wet(100% pick-up of water) rubbing cloth. The diameter of therubbing finger was 16 mm with a downward force of 9 N. Thetest cloth was removed from the finger and air-dried. Thestaining of the test cloth was assessed with the grey scaleunder D65 illumination.

Recycling of fibers from worn denim fabrics to a scarf

In comparison to the recycling of self-dyed pre-consumercotton waste, a used denim fabric was dissolved directly in[DBNH] [OAc] and spun into new staple fibers, which werethen processed into a scarf for demonstration purposes. Theindividual process steps are described in the ESI (Fig. S7 andTables S4–S6†).

Results and discussionRaw-material and pretreatment

Discarded textiles can be classified as pre- and post-consumerwaste. The former is produced during the manufacturing oftextiles and represents, for example, excess material resultingfrom cutting clothing patterns. As its properties are usuallywell known, pre-consumer cotton shows a significantly higherrecyclability than post-consumer garments that have under-gone a whole lifecycle. When establishing a recycling strategyfor cotton rich textiles, it is essential to know the origin of the



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product. The molecular properties of pre-consumer cottonresemble virgin one with a high intrinsic viscosity of up to2000 ml g−1. At the same time, these fibers might contain ahigher amount of dyes and finishing chemicals. Wearing andlaundering subsequently decrease the chemicals present inthe textile product, but also affect the macromolecular mole-cular properties of cellulose such as the degree ofpolymerization.10,11 This implies that the cotton’s viscositydecreases dependent on the usage, eventually creating a veryheterogeneous post-consumer product.

In order to study the recyclability of dyed textile waste sys-tematically and to gain a better understanding of the associ-ated challenges, both pre-consumer and post-consumer cottonwere used as raw material. The fabrics where then dyed asdescribed in the Materials and methods section to obtain sub-strates that resemble real waste textiles, but with exact knowl-edge on the type and amount of dye in the fabric.

As reported previously, cotton waste needs to show a limit-ing viscosity of 400–500 ml g−1 to be spun to Lyocell-typefibers using NMMO monohydrate or [DBNH] [OAc] assolvent.16 Therefore, the pre-consumer fabric was subjected todifferent pretreatments, such as electron beam irradiation(e-beam), H2SO4 hydrolysis (A), and combinations of acidwashing with H2SO4 (A) and enzymatic hydrolysis with endo-glucanase (EG), after vat-dyeing with Indanthren Blue BC 3%.The idea of the pretreatments was to adjust the degree ofpolymerization (DP) of cellulose to a target DP to obtain theviscoelastic properties necessary for optimal spinning whilemaintaining the original color of the fabric. Mechanical treat-ments such as disk refining or chemicals that would causebleaching (i.e. O3 or H2O2) were thus avoided.

Sequences of acid washing and endoglucanase treat-ments have yet successfully been employed on other wastematerials.38 Whereas acids most commonly degrade cellulose

to a polydispersity index (PDI) close to 2.0,39 enzymes such asendoglucanase result in a broadening of the molar mass distri-bution and enhance the cellulose’s reactivity towards the dis-solving agent.40 In general, a broad molar mass distribution ofthe cellulose substrate can be beneficial for the spinningprocess because it contains both long chains for fiber strengthand shorter ones to act as fillers.41 Therefore, enzymatic treat-ments are expected to result in a better spinnability than acidtreatments only. A combination of both is done for costefficiency and to remove possible contaminations by metals.Adjusting the viscosity by e-beam irradiation appears moresustainable in terms of wastewater generation and saves costsemerging from drying after wet-treatment. During e-beamirradiation, high-energy electrons (10 MeV) penetrate a stack ofsheets of cellulose materials (fabric) with almost constantenergy. The resulting radicals are statistically distributed inthe material, which leads to a uniform degradation of the cell-ulose chains. Due to unsaturated, stable radicals and the for-mation of labile groups such as carbonyl groups, depolymeri-zation reactions may continue even after the e-beam treatment,especially under alkaline and/or oxidative conditions.42

A summary of the studied treatments and their influenceon the limiting viscosity and molar mass distribution is dis-played in Fig. 2a and Table 1.

The initial viscosity of the Indanthren dyed pre-consumerfabric ranged around 2032 ml g−1 and was decreased bye-beam irradiation, acid treatment (A), as well as by EG-A andA-EG sequences to 545, 454, 496, and 470 ml g−1, respectively.Both e-beam and A only had a minor effect on the polydisper-sity index (PDI) of the samples (2.7 and 2.8). Despite thesimilar PDI, it is evident that A tends to degrade rather longerpolymer chains than shorter ones, which is consequentlyreflected in the lower amount of DP > 2000 chains (29.4%)compared to the statistical cleavage induced by e-beam

Fig. 2 Molar mass distribution of (a) pre-consumer cotton dyed with Indanthren Blue BC 3% before and after electron beam (e-beam), endogluca-nase (EG and acid washing (A), acid washing (A) and endoglucanase (EG) pretreatment, and acid washing only (A) as well as (b) the molar mass distri-bution of post-consumer cotton before and after dyeing with Indanthren, Remazol and Levafix colorants.



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irradiation (37.8%). In contrast, both EG-A and A-EGsequences substantially increased the PDI of the cotton sub-strate from 2.4 to 4.6 and 5.0 also leading to a rise in the lowmolecular weight fractions (DP < 100) with 3.6% and 2.7%,respectively.

The aim for the post-consumer waste was to be usedwithout any pretreatment to exclude possible effects thereof onthe dye structures. All post-consumer samples consisted ofwhite hospital bed sheets supplied by a laundering company.It can be assumed that most samples had a similar lifetime,which resulted in similar molecular properties. In an initialassessment, we determined the limiting viscosities of 50 bedsheets to estimate their overall suitability for dry-jet wet spin-ning in [DBNH] [OAc]. The predefined target range for the visc-osities was 350–550 ml g−1. Samples below or beyond thisrange were not considered for further processing. The viscositydistribution of the analyzed samples is illustrated in Fig. 3.

Only 10% (5) of the bed sheets displayed viscosities below350 ml g−1, while 28% (14) showed values above 550 ml g−1.The remaining 62% (31) of the samples were found adequateto be used in the following experiments.

As stated in the Experimental section, bed sheets of varyingviscosities were then combined to 1 kg batches, and sub-sequently dyed with a selection of Indanthren, Remazol, andLevafix dyes. Their average molar mass distributions and visc-osities can be found in Fig. 2b and Table 1. The viscosities ofthe samples varied from 396 ml g−1 (undyed) to 465 ml g−1

(Levafix Brilliant Red E-4BA).The PDI of the bed sheets dyed with Remazol Black B gran

133% and Levafix Brilliant Red E-4BA was noticeably higherwith 2.8 and 3.0 compared to the undyed reference (2.1) andthe remaining dyed samples (2.1–2.2). From the molar massdistributions, it can be inferred that both samples, RemazolBlack B gran 133% and Levafix Brilliant Red E-4BA, containeda significantly higher amount of high molecular weight frac-tions (23.1% and 29.1%) than the rest of the post-consumerbatches (14.3%–18.5%). Remazol and Levafix are reactive dyes,which covalently bind to cellulose, whereas Indanthren dyesare precipitated on the cotton fiber. An effect on the molarmass distribution caused through dyeing cannot be excluded,but seems unlikely in respect to the obvious heterogeneity ofpost-consumer textile waste.

Dissolution of cotton and dry-jet wet spinning

The formation of cellulosic filaments from ionic liquid solu-tions depends on the liquid thread’s ability to withstand capil-lary and deformation forces.43 During the spinning process,the cellulose solution is extruded through small nozzles,where the cellulose molecules are pre-oriented. Further orien-tation is induced by stretching of the resulting filaments in theair-gap.44 The applied draw subsequently aligns the cellulosemolecules coaxially leading to increased breaking tenacitiesand lower linear densities (i.e. titers), which are required fortextile applications. The stretchability of a polymer solution isyet dependent on its rheological properties. Consequently, thezero shear viscosities η0* and the dynamic moduli (G′, G″) attheir crossover point enable to predict the spinnability of thecotton solutions (cf. Table 2, and for more details Table S2,Fig. S3 as well as Fig. S4 in the ESI†).

Table 1 Summary of pre- and post-consumer samples studied including their pretreatments, intrinisic viscosities (ml g−1), number average molarmass Mn (kDA), weight average molar mass (kDA), and poly dispersity index (PDI), as well as high and low molecular weight fractions of cellulose(DP < 100, DP > 2000, %)

Sample PretreatmentViscosity/(ml g−1) Mn/kDa Mw/kDa PDI DP < 100/% DP > 2000/%

Pre-consumerUntreated — 2032 ± 50 423.1 ± 1.0 1417.3 ± 57.7 2.4 ± 1.4 0 78.1 ± 0.8Indanthren Blue BC 3% e-beam 545 ± 11 147.3 ± 3.2 391.1 ± 10.6 2.7 ± 0.1 0.3 ± 0.1 37.8 ± 0.4Indanthren Blue BC 3% EG-A 496 ± 15 67.1 ± 7.6 304.5 ± 13.4 4.6 ± 0.7 3.6 ± 1.3 24.8 ± 0.5Indanthren Blue BC 3% A-EG 470 ± 12 75.9 ± 1.2 380.0 ± 55.9 5.0 ± 0.8 2.7 ± 0.2 28.1 ± 0.5Indanthren Blue BC 3% A 454 ± 11 114.5 ± 14.5 314.9 ± 6.1 2.8 ± 0.4 0.9 ± 0.9 29.4 ± 0.7

Post-consumerUndyed — 396 ± 16 94.1 ± 4.2 193.8 ± 2.6 2.1 ± 0.1 1.5 ± 0.4 15.3 ± 0.2Indanthren Brilliant Red FBB — 461 ± 78 93.7 ± 1.5 206.5 ± 3.5 2.2 ± 0.02 1.4 ± 0.06 17.7 ± 0.3Indanthren Brilliant Green FBB Coll — 435 ± 33 100.1 ± 4.0 210.5 ± 1.0 2.1 ± 0.1 1.3 ± 0.3 18.5 ± 0.02Remazol Black B gran 133% — 448 ± 113 91.8 ± 0.1 255.3 ± 0.1 2.8 ± 0.003 1.9 ± 0.08 23.1 ± 0.2Remazol Brilliant Blue R spec — 422 ± 63 91.9 ± 2.5 204.0 ± 0.5 2.2 ± 0.1 1.8 ± 0.2 17.5 ± 0.04Levafix Brilliant Red E-4BA — 465 ± 28 102.4 ± 5.3 308.3 ± 1.5 3.0 ± 0.1 1.6 ± 0.4 29.1 ± 0.2Levafix Blue E-GRN gran — 426 ± 56 89.1 ± 1.8 186.8 ± 0.3 2.1 ± 0.04 1.7 ± 0.3 14.3 ± 0.01

Fig. 3 Viscosity distribution of the 50 hospital bed sheets analyzed; inred, the number of samples chosen for dry-jet wet spinning.



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Viscoelastic solutions usually show a Newtonian plateau atlower shear rates until shear thinning occurs due to increasedshear stress.45 Supposing that the Cox Merz rule is valid, thisbehavior allows the calculation of the zero shear viscosity η0*using the Carreau or the Cross model.36 η0* denotes the vis-cosity of a polymer solution upon no shear stress applied. It isdirectly affected by the concentration and temperature withinthe spinning process. In previous studies, we could show thata η0* between 20 000–30 000 Pas indicates the optimum spin-ning temperature if standard dissolving pulp is employed.35,46

However, these values deviate once waste materials such aspaper or cotton are dissolved.16,38,47

As depicted in Table 2, the average zero shear viscosities ofthe pre-consumer samples ranged between 16 400–31 100 Pasat 70 °C, while η0* for post-consumer cotton was significantlyhigher with 32 700–78 200 Pas at optimum spinning tempera-tures around 65 °C. This demonstrates that pre- and postcon-sumer cotton waste show different ideal spinning tempera-tures and zero shear viscosities dependent on origin and pre-treatment, and might thus demand different processingconditions.

Presumably, this deviation is due to a difference in themolar mass distribution of the samples. The crossover point(COP) of storage and loss modulus (G′, G″) is influenced by theamount of long and short polymer chains present in the spindope, as well as by the overall viscosity of the cellulose sample.The narrower the molar mass distribution, the lower is thesubstrate’s polydispersity index, and the higher is the COP(G′ = G″). Moreover, lower limiting viscosities of the cellulose pulpshift the COP to higher angular frequencies ω.48 In particular,the enzyme pretreated pre-consumer cotton (Indanthren BlueBC 3% A-EG and Indanthren Blue BC 3% EG-A) displayednoticeably lower average values for the G′ = G″ (1630–2179 Pa)than the remaining pre- and postconsumer samples(3131–6996 Pa). These results correspond to the molar massdistributions depicted in Table 1 and in Fig. 2a & b. Inevitably,these properties affected the spinnability of the respectivecotton solutions, which is reflected in the fiber yields summar-ized in Table 2.

The highest yields could be obtained with pre-consumerIndanthren Blue BC 3% EG-A (20–74%). Neither electron beamradiation nor A led to quantitative fiber amounts (0–25%). It isunclear whether this discrepancy can solely be attributed tothe narrow molar mass distribution,41 or to a lack of optimiz-ation. Despite similar COPs, all spinnable post-consumersamples resulted in yields between 34–49% at 5 °C lower spin-ning temperatures. An exception was Levafix Brilliant RedE-4BA, which was not spinnable at all because of its obviousgel character implied by the absence of the COP. In thiscontext it also needs to be noted that fiber yields can only beregarded as tentative indicators for a substrate’s spinnabilityas the whole study was conducted on a lab-scale equipment,where material losses cannot be avoided because of batch pro-cessing. Despite the high yields reached with the EG-Abatches, it is evident that the post-consumer samples showedhigher yields on average. The spinning quality would thus T

able

2Su

mmaryofthenumberofsp

inningtrials

persample,fiberyields(%

),co

ttonco

nce

ntrations(w

t%)ofthesp

indope,averagerheological

properties,

such

asze

rosh

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viscosity

η 0*

(Pas)

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amic

moduliat

theircrossove

rpoint(G

’=G’’,Pa)

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ω(1

s−1 )an

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T(°C),titers

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reak

ingtenac

ities(cN/tex

),an

delongation

atbreak

(%)ofthefibers

spunwithdraw

ratio(D

R)of12

inco

nditionedan

dwetstate,a

swellas

thebirefringence

valueΔnan

dtotalo

rientationf totalofthefibers

Sample

Trials

Cotton/

wt%

Spin

dop

erheo

logy

DR

Fibe

rprop

erties

Con

dition

edWet

Birefringence

T/°C

η 0*/Pa

sω/(1s−

1)

G′=G″/Pa

Fibe

ryield/

wt%

Titer/

dtex

Tenacity/

(cN/tex)

Elongation

/%

Tenacity/

(cN/tex)

Elongation

/%

Δn

f total

Pre-consu

mer

Inda

nthrenBlueBC3%

e-be

am3

13–14

7031

100

1.07

6996

120–

171.4±0.2

40.3

±2.7

7.4±1.4

33.7

±3.3

7.25

±1.03

0.04

8±0.00

70.78

±0.12

Inda

nthrenBlueBC3%

EG-A

1213

–14

7031

100

0.53

2179

1220

–74

1.2±0.1

44.3

±1.4

10.8

±0.6

39.5

±3.2

12.5

±0.7

0.04

8±0.00

70.78

±0.11

Inda

nthrenBlueBC3%

A-EG

113

7016

400

0.61

1630

1213

1.0±0.1

45.5

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8.9±1.1

45.0

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11.5

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0.04

4±0.01

70.72

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Inda

nthrenBlueBC3%

A2

1370

2650

00.90

3905

120–

251.2±0.1

48.5

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12.8

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47.4

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12.4

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4±0.00

40.71

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Post-con

sumer

Undy

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1365

4750

00.42

4554

1234

1.2±02

56.4

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13.0

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53.4

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13.7

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0.04

3±0.00

40.70

±0.07

Inda

nthrenBrillian

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113

6537

200

0.76

4913

1239

1.4±0.4

54.4

±8.4

14.3

±1.0

54.3

±2.3

14.3

±1.1

0.04

5±0.00

60.72

±0.09

Inda

nthrenBrillian

tGreen

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113

6541

200

0.37

3484

1242

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54.7

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12.3

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51.2

±6.0

13.2

±1.5

0.04

4±0.00

50.71

±0.09

Rem

azol

Black

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133%

113

6578

200

0.29

4319

1242

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50.7

±8.8

13.1

±1.4

51.4

±2.6

14.2

±1.5

0.03

8±0.01

00.61

±0.16

Rem

azol

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tBlueRsp

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5590

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1249

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59.8

±4.1

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57.0

±4.4

15.1

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0.04

7±0.00

40.75

±0.07

LevafixBrillian

tRed

E-4BA

113

6581

220

0n.a.

n.a.

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n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

LevafixBlueE-GRNgran

113

6532

700

0.51

4317

1243

1.2±0.2

56.0

±4.7

13.1

±1.9

54.3

±4.3

13.8

±1.8

0.04

7±0.01

50.76

±0.22



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better be described by the maximum draw ratio extruded fila-ments can withstand; its assessment was however without thescope of this study.

The resulting fiber properties (see Table 2 and Fig. 4) showa considerable difference between the spun pre- and post-con-sumer samples. As mentioned in the experimentals, all fiberswere stretched with a draw ratio of 12, which means that thetake-up velocity of the godets collecting the fibers is 12 timeshigher than the extrusion velocity of the spin dope. At similarconcentrations (13–14 wt%), this leads to fibers with compar-able linear densities (1.0–1.4 dtex). The variation within thisparameter usually results from differences in the solubility ofthe cotton pulp during the dissolution stage, which finallyinfluences the real concentration of the cotton solution. It wasobserved that untreated cotton pulp dissolved better than pre-treated one, which formed undissolved aggregates in the spindope presumably due to hornification after wet processing anddrying.

The breaking tenacities and elongations of the spun post-consumer cotton (50.4–59.0 cN/tex, 12.3–14.3%) in con-ditioned state were significantly higher than the values of thepre-consumer samples (40.3–48.5 cN/tex, 7.4–12.8%). Thistrend also continued in wet state (51.2–54.3 cN/tex, 13.2–14.2%for post-consumer; 39.5–47.4 cN/tex, 11.5–12.5% for pre-consu-mer), and is reflected in the Young’s Modulus of the fibers(see Fig. 4 and Table S3,† as well as average stress strain curvesin Fig. S5 & S6†). A similar trend for the total orientation ofthe fibers could not be observed (cf. Table 2).

Although all fibers exceeded the properties of commercialLyocell and Viscose, the recycled post-consumer fibersappeared on average stronger and more flexible than the onesproduced from pre-consumer waste, exhibiting Young’sModuli from 12.0–14.4 GPa and 13.1–17.0 GPa, respectively.Moreover, the post-consumer samples yielded also 6–25%better tenacities compared to the Ioncell fibers spun frombleached pre-hydrolysis kraft pulp. This also corresponds to

the Moduli of Toughness obtained for all regenerated post-consumer fibers displaying values of 60.0–69.5 MPa, whereasthe pre-consumer samples only yielded moduli up to 53.6 MPa(cf. Table S3 in the ESI†).

These results agree with earlier studies describing the re-cycling of cotton waste in NMMO or [DBNH] [OAc].16,17,20 Thechoice of the dye did not appear to have an impact on the fiberproperties apart from Levafix Brilliant Red E-4BA, which wasnot spinnable due to gel formation.

Yarn spinning

All batches of the spun Indanthren Blue BC 3% dyed pre-con-sumer cotton were combined for the yarn spinning process. Allother samples were processed to yarns separately. The fiberswere spin finished to control their antistatic and cohesion pro-perties in the yarn spinning process. The strength of the spunyarns depends on the fiber strength and the frictional resis-tance to slippage.49 The angle of fiber inclination increaseswith increasing yarn twist. At the same time, the component offiber strength in the yarn axis decreases. Otherwise, the fric-tional resistance between the fiber raises as the yarn twist isincreased until the fiber slippage in the yarn is eliminated. Inthe short-staple yarn spinning, 30–70% of the fiber strengthcan be exploited in the yarn.50 The properties of the spunyarns are summarized in Table 3. Similar to the fiber pro-perties, the yarn spun from Indranthren Blue BC 3% yieldedthe lowest tenacity 25.0 cN/tex with an elongation at break of7.6%, while the dyed post-consumer samples showed tenaci-ties of 28.8–32.7 cN/tex and elongations of 6.7–7.1%.

Fabrics and demonstration products

All fabrics used for the wash fastness and rubbing tests as wellas the demonstration products (see Fig. 5) were knitted from theyarns listed above as discussed in earlier publications.35,51 Thebaby jacket was manufactured using the samples obtained fromIndanthren Blue BC 3%, whereas the scarf was knitted fromRemazol Brilliant Blue R spec dyed samples. The fabrics belowrepresent the remaining samples produced from the recycledfibers on the right and the dyed waste fabrics on the left.

Dyes in the spinning process

Whether the original color of a waste fabric can be reused innew textile fibers depends on a variety of factors. Concerning

Fig. 4 Average breaking tenacities (cN/tex) of the spun fibers in con-ditioned state as a function of the average Young’s Modulus (GPa).Viscose, Lyocell, and Standard Ioncell Fibers were added as a reference.

Table 3 Mechanical properties of spun yarns (single thread)

SampleTiter/tex

Tenacity/(cN/tex)

Elongation/%

Pre-consumerIndanthren Blue BC 3% 17.8 ± 1.5 25.0 ± 4.7 7.6 ± 0.9

Post-consumerIndanthren Brilliant Red FBB 21.7 ± 1.7 28.8 ± 3.8 6.9 ± 0.5Indanthren Brilliant Green FFB Coll 20.8 ± 1.8 28.9 ± 3.6 6.4 ± 0.5Remazol Black B gran 133% 25.3 ± 0.5 29.0 ± 3.4 7.1 ± 0.5Remazol Brilliant Blue R spec 21.2 ± 1.0 32.7 ± 3.9 6.9 ± 0.5Levafix Blue E-GRN gran 21.5 ± 0.9 31.4 ± 4.4 6.7 ± 0.6



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post-consumer waste, the dye needs to exhibit a good colorfastness towards washing and rubbing so that the wastegarment still shows a sufficient color intensity at the end ofits lifecycle. When recycled in a dry-jet wet spinning process,dyes additionally require to tolerate the chemical environ-ment thereof, including elevated temperatures and pH, andshould not change the rheological properties of the spindope.25,52 Indanthren Blue BC 3%, Indanthren Red FBB coll,and Indanthren Brilliant Green FBB coll represent vat dyes,which have already been studied as colorants for spin dyeingin the Lyocell process yielding excellent color fastness.28–30

Their oxidized form is usually insoluble in water, whichmakes it possible to precipitate them on the fiber surfacethrough a sequence of reduction and oxidation reactions. Vatdyes also show a high thermal stability due their aromaticstructure formed by antraquinone units. Likewise, reactivedyes with antraquinone (Remazol Brilliant Blue R spec orLevafix Blue E-GRN gran) have been reported to be particu-larly durable towards laundering, elevated temperatures, andbleaching32,33 especially compared to azo chromophores(Levafix Brilliant Red and Remazol Black B gran 133%).Remazol dyes exist as mono (Remazol Brilliant Blue R spec)and bifunctional colorants (Remazol Black B gran 133%) and

attach to the ionized cellulose via etherification of the vinylsulphone groups.53 Competing hydrolysis reactions canhowever cause a restrained substantivity.54 On the contrary,Levafix dyes form covalent bonds with cellulose through anucleophilic substitution reaction of their halo quinoxalineunits. They contain two reactive groups and may form twodye-fiber bonds. Their reactivity is lower compared todichloro-s-triazine dyes and they might undergo hydrolysisreactions in acidic environment.55 Finally, the stability of adye within a recycling process strongly depends on its chemi-cal structure, the nature of its interaction with cellulose andthe solvent.

In initial dissolution experiments (cf. Table S1 in the ESI†),we found all dyes apart from Indanthren Blue BC 3% andIndanthren Brilliant Red FBB, soluble in [DBNH] [OAc]. Thesame applied for almost all dyed cotton fabrics, excluding thesample dyed with Levafix Brilliant Red E-4BA, where aggrega-tions of undissolved fibers could be observed under the micro-scope. The insolubility of the two Indanthren dyes, however,did not affect the quality of the resulting spin dopes. The dyesand cellulose formed a homogenous solution in the ionicliquid, which led to regenerated fibers that displayed auniform distribution of dye throughout the fiber cross-section.27,30 Only Remazol dyes leached into the spin bathcausing color fading, most likely due to insufficient fixationduring the dyeing step.31 Table 4 summarizes the CIELab para-meters lightness L*, a* (red-green axis), and b*(yellow-blueaxis) as well as the overall color difference ΔE in the CIELabspace. ΔE is a tool to assess the similarity of colors duringquality control. Values of 0–0.5 describe no change, 0.5–1.0 analmost imperceptible change (usually not visible by thehuman eye), 1.0–2.0 a small color difference, 2.0–4.0 a percei-vable difference, and 4.0–5.0 a rarely tolerated change, whereasvalues above 5.0 denote a different color.

All dyes showed a noticeable color difference (ΔE =8.8–25.6), with Indanthren dyes (ΔE = 8.8–14.) and Levafix(ΔE = 9.2) resulting in similar fastness properties. During thedissolution of cotton, [DBNH] [OAc] creates an alkalineenvironment that enables the conversion of vat dyes into theirreduced form. This can induce breakages of the hydrogenbonds between cellulose and the respective dyes, which canaffect the final shade of the regenerated fibers. On the con-trary, an alkaline cleavage of the ether bond between celluloseand the Levafix dyes seems rather unlikely.

Fig. 5 Left: Baby jacket produced from the Indanthren Blue BC 3% yarnknitted in 2 × 1 rib on top and in interlock structure below. Right: Scarfknitted from Remazol Brilliant Blue R spec yarn. Below: Dyed wastefabrics on the left and fabrics made from the remaining recycled fiberson the right.

Table 4 Color difference in the CIELab space (ΔE) of the dyed fabrics and the fibers after spinning

Samples

Waste fabric Recycled regenerated fibers

ΔEL* a* b* L* a* b*

Indanthren Blue BC 3% 69.0 ± 0.3 −4.9 ± 0.3 −22.2 ± 1.1 56.6 ± 1.1 −6.6 ± 0.1 −14.0 ± 0.3 14.9Indanthren Brilliant Red FBB 40.5 ± 1 49.1 ± 0.6 6.9 ± 0.4 39.5 ± 0.8 38.1 ± 0.7 12.9 ± 0.3 12.6Indanthren Brilliant Green FBB Coll 37.0 ± 0.5 −46.9 ± 0.4 −1.4 ± 0.1 34.5 ± 0.4 −38.4 ± 0.1 −0.9 ± 0.1 8.8Remazol Black B gran 133% 18.7 ± 0.3 −5.0 ± 0.1 −16.2 ± 0.2 35.6 ± 1.2 15.6 ± 0.3 6.2 ± 0.2 25.6Remazol Brilliant Blue R spec 43.0 ± 0.6 −1.8 ± 0.1 −39.7 ± 0.3 56.9 ± 3.1 0.7 ± 0.7 −20.7 ± 0.4 23.6Levafix Brilliant Red E-4BA 41.9 ± 0.3 62.0 ± 0.3 0.01 ± 0.2 n.a. n.a. n.a. n.a.Levafix Blue E-GRN gran 37.5 ± 0.9 −1.5 ± 0.1 −31.97 ± 0.3 35.3 ± 1.4 −3.5 ± 0.5 −23.3 ± 0.6 9.2



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The color leaching of Remazol Brilliant Blue R spec andBlack B gran 133% was followed by the largest alterations(ΔE = 23.6; 25.6). As displayed in Fig. 6, the absorbancemaximum of Remazol Brilliant Blue R spec did not experienceany significant shift (590 nm to 587 nm) throughout the spin-ning process, while the absorbance band of Remazol Black B133% shifted from 597 nm to 574 nm. According to the ColorIndex, Remazol Black B 133% (C.I. Reactive Black 5) exhibits arather poor alkali resistance, but it remains unclear whetherthe strong color change can be attributed to a degradation ofthe chromophore in [DBNH] [OAc], or to an insufficientfixation of dye during the dyeing step.

Moreover, the spinning process triggered an increase of b*(yellow-blue axis) in all samples, and raised the lightness L* ofthe Remazol dyed samples, while the remaining samples (i.e.Levaifx, Indanthren) showed a slight decrease of L*. For a*(red-green axis), no particular trend could be observed.

Without a doubt, any recycling process that consists ofreshaping polymeric material also brings about color changes.When cotton is dissolved and subsequently coagulated, its

crystalline structure transforms from cellulose I to cellulose II.Besides, the dissolution and regeneration is also accompaniedby a change of the surface morphology of the regeneratedfibers.16 The result is an increased luster, which for examplebecomes evident in the color of the fibers.

Eventually, it can be assumed that the observed colorchanges are a combination of surface phenomena, insufficientboil-off after dyeing, as well as chemical reactions occurring inthe alkaline environment of [DBNH] [OAc]. Apart fromRemazol Black B 133% and Levafix Brilliant Red E-4BA, alldyed waste fabrics nonetheless translated to brightly coloredfibers that certainly can be employed in textile industry (cf.Fig. 5).

Color fastness to washing and rubbing

The color fastness to washing and color fastness to rubbingare the most typical color fastness tests for textiles. Theypredict how much color fades and stains white garmentsduring the laundering or rubbing. Vat dyes (Indanthren) havegood all-round fastness properties because of their water inso-lubility. Reactive dyes (Levafix and Remazol) show in particulargood wet fastness due to covalent bonding between the dyemolecule and cellulose. Table 5 summarizes the wash fastnessresults of all dyed waste fabrics and the fabrics manufacturedfrom the dry-jet wet spun fibers.

As described before, Remazol Brilliant Blue R spec is amonofunctional vinyl sulphone dye. Although it forms acovalent bond with cellulose, it also undergoes a competinghydrolysis reaction with water during the dyeing process. Thesame applies for Remazol Black B gran, but it shows a higherfixation rate because of two reactive groups. Similarly, thedichloroquinoxaline dyes Levafix Brilliant Red E-4BA andLevafix Blue E-GRN gran can partially hydrolyze during dyeingand this may reduce their wash fastness.55 However, sufficientrinsing and boiling-off after dyeing removes the hydrolyzed

Fig. 6 UV spectra of the spinbath after spinning with Remazol Blue Rspec and Remazol Black B 133%, as well as of the pure dyes in water.

Table 5 Wash fastness of cotton and regenerated cellulosic fabrics according to the greyscale

SampleChange incolour

Numerical rating for staining

Wool Acrylic Polyester Polyamide CottonCelluloseacetate

CottonIndanthren Blue BC 3% 5 5 5 5 5 5 5Indanthren Brilliant Red FBB 4–5 5 5 5 5 5 5Indanthren Brilliant Green FBB Coll 5 4–5 5 5 5 5 5Remazol Black B gran 133% 4–5 5 5 5 5 5 5Remazol Brilliant Blue R spec 5 5 5 5 5 5 5Levafix Brilliant Red E-4BA 5 5 5 5 5 4–5/5 5Levafix Blue E-GRN gran 5 5 5 5 5 5 5

Ioncell (from recycled cotton waste)Indanthren Blue BC 3% 5 5 5 5 5 5 5Indanthren Brilliant Red FBB 5 5 5 5 5 5 5Indanthren Brilliant Green FBB Coll 5 5 5 5 5 5 5Remazol Black B gran 133% 4–5/5R 5 5 5 5 5 5Remazol Brilliant Blue R spec 5 5 5 5 5 5 5Levafix Brilliant Red E-4BA n.a. n.a. n.a. n.a. n.a. n.a. n.a.Levafix Blue E-GRN gran 5 5 5 5 5 5 5



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dyes completely. This study demonstrated an excellent washfastness for both cotton and regenerated cellulosic fabrics,regardless of the dyes employed (cf. Table 5).

Table 6 describes the rubbing fastness of the fabrics beforeand after recycling. The low rubbing fastness of the dyed wastefabrics arises from the superficial presence of the dye. It maycause the formation of heavy shades, inadequate washing atthe end of dyeing, and the coloration of only a few moleculesat the textile-air interface. Besides, it might affect the watersolubility of the dyes and lead to a weak dye-fibre attachmenton the surface layer. Overall, wet rubbing fastness tends toproduce lower results compared to dry rubbing, which is dueto the partly solubilization of the dye and its migration to thesurface of colored fabric. Consequently, the regenerated cell-ulose fibers showed a significantly better rubbing fastness asthe dyes were homogenously distributed in the fiber structureby the spinning process.

Recovery of the ionic liquid [DBNH] [OAc]

An entirely green recycling process demands the processingchemicals to be recyclable in the same manner as the wastesubstrate, which it upcycles. After spinning, the ionic liquid[DBNH] [OAc] accumulates in the aqueous coagulation bath.Therefore, the water must be removed from the ionic liquid toa tolerable residual content, while at the same time accumu-lated decomposition products of the solute must be elimi-nated. These approaches are part of our on-going research andwill be published in more detail soon.

Conclusions

We successfully demonstrated that it is possible to upcycledyed pre- and post-consumer cotton waste to new man-madecellulose fibers retaining the color and having superior pro-perties compared to commercial Viscose or Lyocell. The pro-

perties of the regenerated fibers were shown to be dependenton the origin of the waste substrate and the pretreatmentsemployed. This is reflected in the tenacities and elongations atbreak of 40.3–48.5 cN/tex 7.4–12.8% for pretreated pre-consu-mer waste, and 50.7–59.8 cN/tex and 12.3–14.3% for untreatedpost-consumer cotton. Beside of Levafix Red E-4BA, the dyesdid not affect the solubility of the cotton substrates in [DBNH][OAc]. All translated colors changed throughout the spinningprocess (ΔE = 8.8–25.6) most likely due to alterations in themacromolecular structure of cellulose and leaching of the dyesinto the spin bath (i.e.in particular Remazol Black B 133%).Nevertheless, most samples still displayed bright colors, whichwere suitable for the manufacturing of fabrics and demonstra-tor products. The wash and rubbing fastness increased afterspinning as the colorants were incorporated homogenouslyinto the fiber matrix. Eventually, the recyclability of dyedcotton will demand adequate labelling of the chemicalsinvolved to enable proper reprocessing at the end of its life-cycle. It can be concluded that vat dyes and reactive dyes withanthraquinone units have the greatest potential to be re-usedin dry-jet wet spinning. In terms of sustainability, coloredpost-consumer waste cotton requires less processing than pre-consumer fabrics as its molar mass distribution tends to beadjusted to fit the spinning parameters by wearing and laun-dering. Eventually, the manufacture of a scarf successfullydemonstrated that worn jeans (i.e. denim fabrics) can berecycled into new, high-quality fibers that retain most of theircolor using the Ioncell™ process.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project has received funding from the European Union’sHorizon 2020 research and innovation programme undergrant agreement No. 646226. The authors also gratefullyacknowledge Leena Katajainen (Aalto University) and VTT(Finland) for their support in material pretreatment, as well asKaniz Moriam, Mikaela Trogen, Karl Mihhels, Yibo Ma, SherifElsayed and Hilda Zahra (Aalto University) for assisting infiber spinning and dope preparation.

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Table 6 Rubbing fastness of cotton and regenerated cellulosic fabrics

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Dry Wet

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Recycling of vat and reactive dyed textile waste to new ...

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